Hi and welcome! We've got a 1 1/2 year old black tri Aussie named Eli, and we know all about the Aussie urge to "alert" people to threats outside the door.... Eli used to be terrible about this, especially when we'd travel and stay in hotel rooms where unfamiliar noises were so close to the room. We did this: when Eli first barked, we praised him for doing a "good job" of protecting us. (Aussies love jobs!) Then we gradually taught him the word "quiet" (treats and praise, treats and praise), and he eventually learned that it's okay to bark ONCE but then he must not continue barking.

Your boy Foster is no doubt way-smart and will catch on to the commands easily if you work with him. Use "quiet" along with the finger-to-your-lips signal, and I bet he'll learn pretty fast!

Post a pic when you can, too!